Has the algorithm for detecting spelling mistakes changed?
I've recently noticed that the exercises are a lot stricter on spelling mistakes than they used to be. Typically, you'd get a single spelling mistake wrong as long as it wasn't another word (e.g. sie -> she).
I just typed 'bod' instead of 'bad' for this exercise and got marked wrong: https://www.duolingo.com/comment/787597. It's hard to judge these things, but in my experience, that should have flown.